There doesn't appear to be a reason. Hell, even the 12th, probably among the most specialized of the divisions bar 2nd and 11th (The former of which mainly due to the fact that the captain of the 2nd can ALSO lead the secret corps rather than anything the 2nd does, and the later of which only barely qualifies as 'specialized') is only a recent thing.
Which is part of the reason why, when mentioned a while ago in one of the other threads, I proposed to restructure the entire Gotei from the ground up. 4-5 core divisions with 'commanders', each having several or more subdivisions with a particular bend to them that contributes to a whole, which is where the 'Captains' part would come in, being subdivision heads. Get some goddamn structure in there so it makes sense.
I believe I had it such that Unohana headed both Kido and the healing squads, Shunsui's division was more combat based, so he had to handle Kenpachi and the like, Ukitake was oriented more around administration, so they handled sending people out to the human world and the afterlife stuff, which likely pulls Tousen under his wing with the newspaper he heads.
Yamamoto of course heads all the shit as top dog and his division handled the more governmental stuff, like the science division and secret corps, hence he Soifon and Mayuri are all under one banner to get all the evil people destroyed in on go if desired.
Aizen I had as a possible fifth head to be oriented around a division based around investigations and security, hence Gin and Hitsugaya would be under his command with their own respective subdivisions. Which would naturally utterly wreck the dynamic of this entire division when only one of the subheads remains after the betrayal. :sisi: